Groove Tube GT-6L6R (B) Quartet (set of 4 matched tubes) Russian made, same as the 5881 you are using now, but when these are matched in our system they are improved. These are a very sturdy and reliable tube. Perhaps the most reliable of the currently made types, with slightly less power and a more aggressive edge when pushed into distortion. These are the tubes that are used by Fender in amps that use 6L6 tubes and are amps that DO NOT come from their custom shop. These are used in the Hot Rod series, Blues DeVille series, Pro Tube series, BF reissue series. The GT 6L6GE is used in the 57 Tweed, Vibroverb and Vibro King amps. If you are trying to match your stock tube rating with the GT rating: On Fender amps, output tubes in newer amps are color coded. The scale is: Blue = GT 1-3 rating White = GT 4-7 rating Red = GT 8-10 rating Please specify if you need to order a rating.